A pervasive trend in modern integrated circuit manufacture is to increase the amount of data stored per unit area on an integrated circuit memory unit, such as a flash memory unit. That is, as flash memory technology progresses, the speed and memory density becomes higher and higher. Modern flash memory units are characterized by the non-volatility of the charge stored in the arrays of memory cells that make up the memory unit.
Memory units often include a relatively large number of core memory devices (sometimes referred to as core memory cells). These core memory devices can comprise a floating gate device where a conductive charge storing region (or floating gate) is located between a conductive wordline and a channel region of a substrate. The channel region is laterally disposed between a pair of bitlines. The floating gate can be separated from the wordline and the channel region by respective dielectric layers. In an alternative arrangement, the floating gate can be replaced by a non-conductive charge storing layer that can store data in plural charge storing regions. For example, a normal bit can be stored using a charge storing region adjacent a first bitline associated with the memory device and a complimentary bit can be stored using a charge storing region adjacent a second bitline associated with the memory device.
Programming of the foregoing memory devices can be accomplished, for example, by hot electron injection. Hot electron injection involves “pulsing” the device by applying appropriate voltage potentials to each of the wordline (the wordline connected to or defining a control gate of the memory device) and a drain of the memory device for a specified duration. During the programming pulse, the source can be grounded or coupled to a bias potential to assist in controlling the amount of charge injected into the memory device. Reading of the memory device can be accomplished by applying an appropriate voltage to each of the gate, source and/or drain and comparing the drain to source current (as an indication of device threshold voltage) against a reference value to thereby determine if each memory cell is in a programmed or an unprogrammed state.
Even though conventional charge trapping flash memory units are capable of storing a relatively large amount of data in a relatively small area, there is an ever increasing demand to store even more data per unit area of a memory core and for methods of increasing the speed with which the data can be programmed.